The broad objectives of the proposed research are: 1. To gain insight into the molecular events that influence the interaction of the isozymes of aspartate transaminase with other intracellulor macromolecules of heart tissue. 2. To determine the lifetimes in serum of each isozyme in healthy animals and those with heart disease. 3. To develop a reliable, fast and simple assay for the quantitation of the iaozymes when both are present in serum or in tissue specimens. 4. To explore the role of certain drugs in their interaction with each isozyme and the effect of these interactions on transaminase-protein complexes in order to probe for causes that affect release of a given isozyme from heart. 5. To obtain kinetic and structural information on the properties of specialized regions of the isozymes. 6. To detect possible site-site cooperativity between the active sites of each isozyme in order to understand possible catalytic regulatory mechanisms. 7. To assimilate all structural, kinetic and macromolecular interaction data to produce a molecular model for the different function and/or regulation of each isozyme which would explain the functional need for heart tissue to produce two proteins with the same overall catalytic purpose. BIBLIOGRAPHIC REFERENCES: B. Boettcher and M. Martinez-Carrion (1975) Biochemistry 14, 4528 "Hybridization of Glutamate Aspartate Transaminase. Investigation of Subunit Interaction". M. Martinez-Carrion, B. Boettcher and G. Churchich (1975) Biochem. Biophys. Res. Commun., 67, 1480. "Fluorescence Energy TRansfer between Active Sites in Aspartate Transaminase".